Pre-Dalradian basement, Central Highlands (Grampian) Terrane, Northern Ireland

Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.

M R Cooper and T P Johnston

Pre-Dalradian Basement

Distribution of Pre-Dalradian Basement and Dalradian rocks in Northern Ireland. (P947791)

Geological map of the Pre-Dalradian Basement and Dalradian rocks in Co. Fermanagh and south Co. Donegal (16). (P947792)

Banded gneissose psammite of the Lough Derg Group. Croagh mountain [G 985 630], 6 km northeast of Belleek, Co. Fermanagh. (Pen 15 cm long). (P947959)

Metabasite pod in gneissose psammite of the Lough Derg Group. Croagh mountain [G 985 630], 6km northeast of Belleek, Co. Fermanagh. (Hammer 36 cm long). (P947960)

The oldest rocks in Northern Ireland occur in two fault-bounded inliers in Counties Fermanagh and Tyrone and are tentatively assigned to the Moine Supergroup. In Co. Fermanagh the Lough Derg Group is part of the stratigraphical sequence in the Lough Derg Inlier while in Co. Tyrone the outcrop of the Corvanaghan Formation is restricted to the Central Inlier (P947791). Their precise age is not known but because the rocks have experienced higher grade metamorphism than the Dalradian rocks in Northern Ireland they are presumed to be significantly older. The evidence for a high pressure-high temperature (granulite facies) metamorphic event coupled with a radiometric date of 895±60Ma (Rb-Sr whole-rock isochron) points to a Grenvillian age for the rocks . In both inliers there is evidence of polyphase deformation and a high-grade regional metamorphic event overprinted by a later (Caledonian) retrogressive event.

Lough Derg Inlier (Lough Derg Group: Slishwood Division)

The Lough Derg Inlier straddles the Fermanagh-Donegal border between Lough Derg and Lower Lough Erne (P947792). Carboniferous rocks lie unconformably on the Lough Derg Group along its western and southern margins. On its southeast side the Pettigoe Fault juxtaposes the inlier with Carboniferous rocks in the Kesh-Omagh area while the Lough Derg Slide has transported inverted Dalradian Southern Highland Group rocks southwestwards across the north of the inlier in south Co. Donegal.

The Lough Derg Group consists of medium- to coarse-grained, grey, buff and pink siliceous, feldspathic and micaceous psammites with mica schist bands (P947959). Although locally gneissose and with dominantly granoblastic textures, indicative of considerable deformation and recrystallization, the psammites still retain evidence of original sedimentary structures including cross laminations and slump features. In many exposures, micaceous laminae often impart a flaggy character to the psammites and probably lie parallel to bedding.

The psammites in the Lough Derg Inlier are cut by metabasite dykes up to several metres wide and contain discrete metre-wide pods or trains of boudins parallel to the regional bedding/schistosity (P947960). Mineralogically, the metabasite bodies are amphibolites with at least 75% hornblende and represent basic magma that was intruded into the sediments prior to the main deformation .

Irregular and normally lenticular quartz-feldspar pegmatite veins are late-stage features that transect most other major structures and lithologies in the Lough Derg Inlier. They are composed of quartz, alkali-feldspar (microcline), biotite and muscovite and are up to 3 m wide such as at Larkhill [H 013 633] (P947792). Locally, the veins contain up to 65% modal potash feldspar, which in the 19th Century provided raw material for the manufacture of porcelain in the Belleek Pottery .

The rocks of the Lough Derg Group contain evidence of a complex deformational and thermal history during which an early formed gneissose banding was subjected to polyphase folding and deformation. During the Caledonian Orogeny the rocks were subsequently folded into a regional antiform plunging northeastwards . There is also evidence of at least two major metamorphic events. An early, possibly Grenvillian, event is represented by granulite facies mineral assemblages but is overprinted by younger retrogressive amphibolite facies mineral assemblages of probable Caledonian age . Current stratigraphical correlation, supported by radiometric dates, indicate that the Lough Derg Group is part of the Slishwood Division that includes some of the rocks of the Ox Mountains Inlier, located to the southwest .

Central Inlier (Corvanaghan Formation)

The Central Inlier in Co. Tyrone (P947791) is a fault-bounded fragment of pre-Caledonian Proterozoic continental crust that is surrounded by Ordovician rocks of the Tyrone Igneous Complex (see Midland Valley Terrane article). While the bounding faults appear to have mainly transverse movements the northeast margin of the inlier is delineated by a low-angle fault which thrusts rocks of the Corvanaghan Formation northeastwards onto the Tyrone Plutonic Group.

The Central Inlier consists of gritty, medium- to coarse-grained, grey and pinkish grey psammite with minor schistose and pelitic rocks of the Corvanaghan Formation. They are exposed in Corvanaghan quarry [H 718 812] 10 km WNW of Cookstown. Coarse lithologies commonly display a gneissose texture with pink (K-feldspar) and grey (quartz) banding and segregation. The rocks have undergone at least three separate fold phases with the major regional structure comprising an eastward-plunging, open antiform . Mineral assemblages indicate peak metamorphism at sillimanite grade.

The age and stratigraphical position of the Corvanaghan Formation are uncertain and on early geological maps the rocks of the Central Inlier were not distinguished from the Dalradian rocks of the Sperrin Mountains . However, based on the evidence of high metamorphic grade and an unpublished Rb-Sr date of 640Ma (D H W Hutton pers. com.) for muscovite and feldspar in late-stage, cross-cutting pegmatite veins, an early Neoproterozoic or older age is now believed to be more likely.